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XXVTL Electro-optical Investigation of Polarized Light, 

 By J. Elster and H. Geitel*. 



THE photo-electric current produced in an attenuated gas 

 by illumination of the kathode has been shown to be 

 dependent upon the inclination of the vibrations of light to 

 the plane of the kathode, and to attain a maximum when the 

 plane of polarization of the light is at right angles to the 

 plane of incidence, and a minimum for the position at right 

 angles to thisf. 



The further examination of this phenomenon proceeds in 

 two directions : — We may ask, " According to what law does 

 the photo-electric current vary when the plane of polarization 

 of the incident light is made to rotate about the ray as its 

 axis ? " and we may inquire u how the intensity of this current 

 depends upon the angle of incidence of the light." 



We confine ourselves here for the most part to the first of 

 these questions J: towards the solution of the second (which 

 offers greater difficulties) we can here only make some small 

 contributions. 



As we have previously remarked, we are led by the diffi- 

 culties which present themselves in the production of polarized 

 ultra-violet light, to choose the fluid alloy of sodium and potas- 

 sium as the photo-electric sensitive surface in an atmosphere 

 of a rarefied indifferent gas, which permits the use of light 

 from the region of the visible spectrum. But this involves 

 the necessity of enclosing the metallic surfaces subjected to 

 experiment in glass vessels. It would be of advantage that 

 the polarized beam of light should enter the vessel normally, 

 through a glass plate with parallel plane surfaces. We should 

 thus avoid all change of intensity in the exciting beam, which 

 with oblique incidence against the glass wall is associated with 

 change of azimuth. But the insertion of such plate-glass 

 " windows " in the glass vessels involves the use of some 

 cement, which must be of such a nature that in presence of 

 vapour of the alkali metals in a vacuum it shall neither 

 evaporate nor undergo any chemical change. We have not 

 succeeded in finding a cement that will stand under these 

 conditions. Organic substances such as resins are out of the 

 question, since they contain volatile constituents which con- 

 dense upon the kathode, forming a layer almost insensitive to 

 light. And such inorganic substances as potassium and sodium 



* Translated from Ann. Phys. Chem. Bd. lv. Communicated by the 

 Authors. 



t Elster and Geitel, Sitzber. Berl. Akad. Wiss. vi. p. 134 (1894), and 

 Wied. Ann. Hi. p. 440 (1894). 



% These results have been published in part in Sitzber. der Kgl. Acad. 

 Berlin, xi. p. 209(1895). 



